1. Field of the Invention
The present invention relates in general to projection systems, and more particularly to the use of multiple illumination sources for illuminating a phosphor color wheel in a projection system.
2. Description of the Related Art
It is known in the art to produce multi-colored light from a single light source. For example, a phosphor color wheel may be used to produce colored light by shining light of a first wavelength on a plurality of different regions of the wheel, where at least one of these regions comprises a phosphor that fluoresces at a second wavelength when illuminated with light of the first wavelength. By spinning the wheel relative to the stationary light source so as to sequentially illuminate the different regions of the wheel, different colors are produced by the wheel.
Conventional phosphor color wheels comprise a red, green, and blue segments that are either transmissive (i.e. colored filters) or reflective. As an example, where blue excitation light is used, one region of the wheel may be made transparent (or reflective), while the two other regions contain phosphors for absorbing the blue excitation wavelength and re-emit light at red and green wavelengths, respectively. Thus, only a fraction of the light produced by the light source is transmitted through the color wheel. The amount of light that can be generated by a phosphor color wheel is therefore restricted by the flux limit of excitation light incident on the wheel, and by the efficiency of the wheel in converting that light to other wavelengths. The lifetime of the wheel is compromised by the fact that the phosphor degrades quickly if it is not kept within a predetermined temperature range, which in turn limits the intensity of incident light on the wheel
Color wheels have a myriad of applications, such as set forth in U.S. Pat. No. 2,416,301, issued Feb. 25, 1947 to Columbia Broadcasting System; U.S. Pat. No. 7,651,243, issued Jan. 26, 2010 to Optical Research Associates; and pending Patent Application Publication US2012/0201030, filed Feb. 6, 2012, and assigned to Intematix Corporation.
One such application is for generating multi-colored light in color digital projectors. Digital Light Processing (DLP) refers to projector technology that uses a digital micromirror device (DMD) to project an image onto a screen. A typical DLP projector includes a lamp, an illumination system, and a light engine comprising a DMD in combination with a spinning phosphor color wheel. For single-chip DMD light engines, the color wheel has different color filter segments (e.g. absorption filters or interference filters) such that when light illuminates the spinning color wheel different wavelengths of light (colors) pass through the wheel and onto the DMD at different times over the course of one rotation of the wheel.
The DMD is a micro-electro-mechanical system (MEMS) device consisting of a large array of microscopic mirrors that modulate light by independently flipping each mirror through a predetermined angle. The DMD modulates light by turning the mirrors on and off several times during a video frame. A frame is divided into approximately 20 to 60 bit planes of different duration, based on bit sequence and frame rate. During a given bit plane each pixel on the screen is controlled by a single bit and is either driven ‘ON’ or ‘OFF’ for the entire duration of the plane. The number, duration, and location of the “ON” times are adjusted with respect to the timing of the frame for controlling the light level. Combining the ‘ON’ times for a given pixel gives the pixel its proper intensity.
Examples of traditional filter wheels (as opposed to phosphor color wheels) are set forth in U.S. Pat. No. 7,862,182, “OPTICAL SYSTEM FOR A PROJECTOR, AND CORRESPONDING PROJECTOR” (Thollot et al.), U.S. Pat. No. 7,871,168, “ILLUMINATION SYSTEM FOR DUAL-LAMP PROJECTOR” (Liu et al.) and U.S. Pat. No. 6,147,720, “TWO LAMP, SINGLE LIGHT VALVE PROJECTION SYSTEM” (Guerinot et al.).
The afore-noted U.S. Pat. No. 7,862,182 describes a color wheel for use with polarization-based liquid crystal imagers, as opposed to DLP projectors. FIG. 3 of the '182 patent shows a reflective embodiment (i.e. using LCOS devices), while FIG. 4 of the '182 patent shows a transmissive embodiment (i.e. using an LCD). Liquid crystal imagers require the use of polarized light, as discussed above, which does not work well with DLP technology.
In applications such as image-tiling, the images from two or more projectors are projected so as to be adjacent or to slightly overlap thereby creating a larger composite ‘tiled’ image. In such multiple projector systems, with individual un-synchronized light sources and color wheels, highly undesirable variations in color and/or brightness can arise between adjacent projectors as a result of different color balances between the primary colors and the independent light sources lagging each other by unpredictable and variable amounts.